BACKGROUND OF THE INVENTION
The present invention relates generally to variable displacement gear pumps. More particularly, the present invention relates to a variable displacement internal gear pump with pressure compensation.
As will be appreciated by those skilled in the art, fixed displacement gear pumps are widely used because they are simple, rugged, compact, and relatively inexpensive. However, constant pressure systems that use such pumps waste energy by exhausting excess flow at system pressure through relief valves. If gear pumps can be economically made into variable displacement forms, they can be used to make constant pressure systems more efficient.
Gear pumps are made in both external and internal configurations. Internal gear pumps are of two types, internal spur gear or gerotor. Internal spur gear pumps use a crescent shaped member in the space in between the inner and outer gear teeth while gerotor pumps have a tooth profile which does not require a crescent member. The gerotor mechanism is made up of inner and outer toothed elements. The internal toothed element has one less tooth than does the outer element and the outer element uses a conjugate tooth profile. As a result, the inner and outer tooth profiles maintain continuous fluid tight contact during operation.
Designing a pressure compensated variable displacement gear pump is a challenging problem that several engineers have attempted to solve. Most designs involve internal gear arrangements. During the last twenty years, several patents have been issued on such concepts. For example, U.S. Pat. No. 5,476,374, issued to Langeck on Dec. 19, 1995, describes an axially-ported variable volume gerotor pump configuration. In that invention, variable flow control is achieved by returning part of the output flow to the pump inlet. U.S. Pat. No. 4,492,539, issued to Specht on Jan. 8, 1985, shows a variable displacement gerotor pump. The described design varies the eccentric position of an outer member relative to an inner member by rotating position control members. Another patent by Specht, U.S. Pat. No. 4,413,960, issued Nov. 8, 1983, describes a position controlled device for a variable delivery pump. In this patent, the pump body can be rotated through an infinite range of angles relative to the pump housing to regulate the eccentric position of pumping elements. This action controls the volume output of the pump.
Another internal gear pump is shown in U.S. Pat. No. 4,097,204, issued to Palmer on Jun. 27, 1978. The Palmer patent shows a variable displacement gear pump which uses a radial movement of the external gear axis to form an eccentric with the internal gear to vary the volume of fluid displaced by the pump.
While the known art shows variable displacement pump forms that may be physically realized, their complexities make practical commercialization unrealistic. Thus, known art fails to address the need for an improved pressure compensated variable displacement internal gear pump. In particular, the known art fails to provide an internal gear pump using variable displacement that is fast-acting. An improved internal gear pump should quickly respond to changing displacement requirements to improve overall pump efficiency.